1. Field of the Invention
The present invention relates to a red phosphorescent compound and an organic electroluminescent device using the same.
2. Discussion of the Related Art
In general, when electric charges are injected into an organic light-emitting layer formed between an electron injecting electrode (cathode) and a hole injecting electrode (anode) of an organic electroluminescent device, electrons combine with holes to create electron-hole pairs, which then decay to emit light.
Organic electroluminescent devices have advantages in that they can be fabricated on flexible transparent substrates (e.g., plastic substrates) and can be operated at a voltage (e.g., 10 V or below) lower than those required to operate plasma display panels (PDPs) and inorganic electroluminescent devices. Other advantages of organic electroluminescent devices are relatively low power consumption and excellent color reproduction.
Further, since organic electroluminescent (EL) devices can emit light of three colors (i.e., green, blue and red), they have been the focus of intense interest lately as next-generation full color display devices.
A general method for fabricating organic EL devices will be briefly explained below.
First, an anode electrode is formed on a transparent substrate.
Indium tin oxide (ITO) is generally used as the anode electrode.
Subsequently, a hole injecting layer (HIL) is formed on the anode electrode. Copper (II) phthalocyanine (CuPc) is mainly used as a material of the hole injecting layer. The hole injecting layer (HIL) is formed to a thickness of about 10 to about 30 nm.
Then, a hole transport layer (HTL) is formed on the hole injecting layer.
The hole transport layer is formed by depositing 4,4′-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (NPB) to a thickness of about 30 to about 60 nm on the hole injecting layer.
An organic light-emitting layer is formed on the hole transport layer.
If necessary, a dopant may be added to a material for the organic light-emitting layer.
For red phosphorescence emission, 4,4′-N,N′-dicarbazole-biphenyl (CBP) as a material for the organic light-emitting layer is deposited to a thickness of about 30 to about 60 nm on the hole transport layer, and an iridium complex is mainly used as the dopant.
An electron transport layer (ETL) and an electron injecting layer (EIL) are sequentially formed on the organic light-emitting layer. Alternatively, an electron injecting/transport layer is formed on the organic light-emitting layer.
Tris(8-hydroxy-quinolate)aluminum (Alq3) is mainly used as a material of the hole transport layer.
Then, a cathode electrode is formed on the electron injecting layer, and finally a passivation film is formed thereon.
Blue, green and red organic electroluminescent devices can be realized, depending on the formation method of the light-emitting layer.
In the light-emitting layer, holes injected from the anode electrode are recombined with electrons injected from the cathode electrode to form excitons.
The excitons are composed of singlet excitons and triplet excitons present in a ratio of 1:3. Only singlet excitons are used in fluorescence, whereas both singlet excitons and triplet excitons are used in phosphorescence processes to exhibit higher luminescence efficiency.
In particular, the quantum efficiency of red phosphorescent materials is considerably high, compared to that of fluorescent materials. Accordingly, a number of studies associated with the use of red phosphorescent materials in organic electroluminescent devices are being made to enhance the efficiency of the organic electroluminescent devices.
Luminescence efficiency (ηle) is represented by the equation below:ηle=k·ηint·ηout 
wherein k is human color sensitivity, ηint is internal quantum efficiency and ηout is outcoupling efficiency.
In order to obtain high external quantum efficiency, phosphorescent materials for use in organic EL devices must satisfy the requirement of high internal quantum efficiency. However, as shown in FIG. 1, as the color purity of an organic EL device using a red phosphorescent material increases as the x-values on CIE chromaticity coordinates increase), the relative spectral sensitivity of the organic EL device decreases, making it difficult to achieve external quantum efficiency comparable to internal quantum efficiency.
Accordingly, there is a demand for development of a red phosphorescent compound that exhibits high color purity (CIE color purity X≧0.65), high luminescence efficiency, and long luminescence lifetime.